The present invention relates to an improved continuous process for the preparation of anhydrous solutions of perpropionic acid in benzene, starting from hydrogen peroxide and propionic acid.
Percarboxylic acids have become increasingly important in the reaction of olefines to give epoxides (D. Swern, "Organic Peroxides", Wiley 1971, Vol. II, page 360 II.) and of cyclic ketones to give lactones (Houben-Weyl "Methoden der organischen Chemie" (Methods of Organic Chemistry) Volume IV/ 2, page 708). However, the aqueous solutions of percarboxylic acids with 2 - 4 carbon atoms, which are readily accessible, for example according to German Patent No. 1,165,576 and German Patent No. 1,170,926, are not suitable for these reactions in all cases due to the presence of water, since water promotes opening of the ring in the products obtained in the reaction. On the other hand, anhydrous solutions of percarboxylic acids give excellent results (see loc. cit.).
The synthesis of anhydrous or substantially anhydrous solutions of percarboxylic acids in organic solvents is known (see Ullmann, Enzyklopadie der Technischen Chemie (Encyclopaedia of Industrial Chemistry), supplementary volume 1970, Neue Verfahren (New Processes), page 181 et. seq. and Swern, Organic Peroxides I, 1970, page 313 et. seq.). These solutions can be obtained, for example, by autoxidation of aldehydes in an anhydrous medium, for example in carboxylic acid esters.
This method has the disadvantage that explosive intermediate products can form in this process and that the carboxylic acid corresponding to the aldehyde is obtained as a by-product after the reaction of the per-acid, for example with an olefine.
Organic solutions of percarboxylic acids are also obtained by the reaction of hydrogen peroxide with carboxylic acids in the presence of an acid catalyst, according to equation (1) ##STR1## when the reaction is carried out in the presence of an organic solvent and the water is removed by azeotropic distillation (DTAS (German Published Specification) 1,043,316, U.S. Pat. No. 2,877,266 and DTOS (German Published Specification) 1,917,032).
However, it is also possible first to prepare an anhydrous solution of hydrogen peroxide in an organic solvent and then to react this with a carboxylic acid in the presence of an acid catalyst (DTOS (German Published Specification) 2,038,318).
Furthermore, organic solutions of percarboxylic acids can be prepared by discontinuous or continuous extraction of pure aqueous percarboxylic acid solutions with organic solvents, for example carboxylic acid esters, phosphoric acid esters and chlorinated or aromatic hydrocarbons, and subsequent azeotropic dehydration of the resulting extracts, or also solely by azeotropic dehydration of the aqueous percarboxylic acid solutions. In this case, part of the organic solvent employed is used to dilute the percarboxylic acid (DTOS (German Published Specification) 2,141,155, DTOS (German Published Specification) 2,145,604 and DTOS (German Published Specification) 2,141,156).
Other processes for the preparation of anhydrous solutions of percarboxylic acids are based on the extraction, with organic solvents, of the equilibrium mixture which is set up according to equation (1) and which consists of hydrogen peroxide, carboxylic acid, water and percarboxylic acid and which may still contain the acid catalyst (DTOS (German Published Specification) 2,141,156, DTAS (German Published Specification) 1,048,569 and DTOS (German Published Specification) 1,618,625).
In these processes, the extractions to obtain anhydrous percarboxylic acid solutions are carried out either in several stages or in one stage. In every case the raffinate, which, regarded from the total preparation of the percarboxylic acid, still contained considerable amounts of hydrogen peroxide and possibly of the acid catalyst, was discarded and these substances were thus lost.
However, processes have also been disclosed according to which the raffinates obtained can be worked up in order to recover the amounts of hydrogen peroxide or acid catalysts contained therein and to recycle these to the reaction of hydrogen peroxide with carboxylic acid.
Thus, unreacted hydrogen peroxide contained in the raffinate can be destroyed according to known methods and the acid catalyst can be recovered (DTOS (German Published Specification) 2,312,281).
According to another process, aqueous hydrogen peroxide is first reacted, according to equation (1), with a carboxylic acid with 2 - 4 carbon atoms, in the presence of an acid catalyst, it being possible for the molar ratio of hydrogen peroxide/carboxylic acid employed to be 0.5 - 30 : 1. The reaction mixture is extracted in counter-current with an organic solvent and, if appropriate, the extract, which has been further treated with water, is dehydrated by azeotropic distillation. The raffinate obtained from the extraction of the reaction mixture with an organic solvent contains both unreacted hydrogen peroxide and also the acid catalyst. These can be recovered and recycled into the reaction by feeding the raffinate to an evaporator unit and distilling off, under reduced pressure, the water introduced with the starting materials and formed by the reaction according to equation (1) and recycling the raffinate reconcentrated in this way into the reaction of hydrogen peroxide with the carboxylic acid. In some cases it also suffices to work up only a part of the raffinate in the manner described. The hydrogen peroxide consumed after the reaction according to equation (1) is replenished after reconcentrating the raffinate. According to this process, the yields of percarboxylic acids are 87 - 90.5%, relative to hydrogen peroxide employed (DT-OS (German Published Specification) 2,262,970).